-Lets consider this then. So you would start off with limited skills, and would be required to learn skills from Monsters. Ok. So youve just created a class who now learns skills more difficultly than other classes. That means in party play, unless you have all the skills for your lvl, youre gonna be dead weight to the party.
-Then you have to consider. How many skills will you have access too? If you give them access to "as many as they want so long as they learn them," that gives BLUs overwhelming advantages party wise compared to other classes. So youre gonna have to limit what skills they could learn, or how many skills they can use at any given time.
-Then you need to consider balance of skills vs other classes. In solo content, having powerful skills makes sense cause your survival depends on it. In party content, that power is divided up between members. To ensure BLU would be balanced in a party, that means gimping the spells learned from monsters. Youll have your generic dot, generic AoE, Generic single target skills. You may have some skills with CC on them, but theyll be limited in scope because High DMG + CC is OP in a party setting. So what becomes the play mechanic of the class then. It was "Learn all these powerful monster skills." But now thats being ham-stringed by balancing it against a party setting. So it just becomes a caster who has a harder time learning skills and will be essentially more akin to BLM but without any of hte shifting mechanics. And if you add specialty mechanics, then its no longer a BLU, because its core identity is learning skills from monsters.